1. Field of the Invention
The present invention is directed to a power supply, and more particularly to a power supply including a chopper which provides from an AC voltage source a DC voltage with an improved power factor.
2. Description of the Prior Art
Power supplies including a chopper are well know in the art to provide a DC voltage from an AC voltage source for driving a load such as a discharge lamp through an inverter which converts the DC voltage to another high frequency AC voltage. FIG. 1 illustrates a typical prior art power supply which comprises a fullwave rectifier 2 providing a pulsating DC output from the AC voltage source 1 such as AC mains, and a chopper CH providing a step-up DC voltage from the pulsating DC voltage. The chopper CH includes an inductor 4, a MOSFET 5, and a smoothing capacitor 7. MOSFET 5 is connected in series with the inductor 4 across the rectifier 2 and is driven to turn on and off for providing a periodically interrupted voltage which is applied through a blocking diode 6 across the smoothing capacitor 7 to provide a resulting smoothed DC voltage to a load 3. MOSFET 5 is driven by a chopper controller 10 comprising an astable multivibrator 11 (.mu.PC1555, available from Nippon Denki Kabushiki Kaisha) for controlling to turn on and off MOSFET 5 at a fixed frequency. The multivibrator 11 receiving an operating voltage at control voltage terminal (pin no. 5) and provides high and low control signal at an output terminal (pin no. 3) at a timing determined by a time constant of resistors 12 and 13 and a capacitor 14. The control signal is fed through a buffer 15 to turn on and off MOSFET 5 at a fixed duty cycle in such a manner as to store energy from the rectifier output when MOSFET 5 is turned on to flow a current through the inductor 4 and MOSFET 5 and to release the energy from the inductor 4 into the capacitor 7 when MOSFET 5 is turned off to allow the current from the inductor 4 to continuously flow into the capacitor 7. The current flowing through the inductor 4 is shown in FIG. 2A. As seen in the figure, when MOSFET 5 is turned on at a time t1, the current responds to increase. Upon MOSFET 5 being turned off at a time t2, the current decreases to zero at time t3 and oscillates around zero level until MOSFET 5 is subsequently turned on at a time t4. This oscillation results from the fact that, as shown in FIG. 1, the inductor 4 will cooperate with parasitic capacitance inherently present in the chopper, i.e., capacitance C1 across MOSFET 5, capacitance C2 across the diode 6, and capacitance C3 across the rectifier 2 to form a resonant circuit. It is thus formed resonant circuit that allows the current to oscillate around zero level and applies a corresponding oscillating voltage to MOSFET 5 as well as diode 6 to give unduly high stress to the components. In addition to this undesired stress, the prior power supply suffers from a problem that as the oscillating current lasts over a longer dead period (t3-t4), an input current from the AC voltage source 1 will have a higher content of harmonics to be distorted thereby by a larger extent. Due to that fact that, when the pulsating DC voltage from the rectifier 2 is high, the inductor 4 sees the supplying current at a high gradient from the rectifier output and flows the releasing current at a low gradient to the smoothing capacitor 7 and that, when the pulsating DC voltage is low, the inductor 4 sees the supplying current at a low gradient from the rectifier output and flows the releasing current at a high gradient to the smoothing capacitor, the above dead period (t3-t4) will be longer at the lower rectifier output than at the higher rectifier output. Consequently, the input current is distorted by the harmonics due to the elongated dead period when the pulsating DC voltage from the rectifier is low, as shown in FIG. 3B, in relation to the input voltage from the AC voltage source, as shown in FIG. 3A, thereby lowering power factor.
In order to avoid the above problem, it is contemplated to eliminate the above dead period by controlling MOSFET 5 of the circuit of FIG. 1 to turn on before the inductor 4 has not released its energy completely, as shown in FIGS. 4A and 4B. However, with this scheme, MOSFET 5 is turned on while the diode 6 flows the current into the smoothing capacitor 7 so that the capacitor 7 acts to apply a reverse voltage to the diode 6. Consequently, a recovery current Ir will flow through diode 6 to cause increase stress thereto. Further, since the current flows constantly through the inductor 4 to continuously increase the energy (L.times.I.sup.2 /2) stored therein, the inductor becomes saturated. In view of these and the above problems, it is therefore demanded to release the energy stored in the inductor before supplying energy thereto and at the same time to minimize the dead period in which the current flowing through the inductor oscillates, particularly at the low level of the pulsating DC voltage from the rectifier.
To this end, it has been proposed another prior art power supply which, as shown in FIG. 5, has a specific controller 40 in addition to a like chopper 30 of an inductor 31, MOSFET 32, blocking diode 33, and smoothing capacitor 34 in order to control MOSFET 32 to turn on immediately after the inductor 31 has released its energy. The controller 40 comprises a flip-flop 41 having its Q output connected to the gate of MOSFET 32 and a current sensor 42 which monitors the current flowing through a current sensing resistor 35 as indicative of the current flowing through the inductor 31 so as to issue a high level output to set input S of flip-flop 41 upon the current through the inductor 31 reduced to zero, thereby giving high level gate signal to turn on MOSFET 32 for storing energy into the inductor 31 by the current flowing from the rectifier 22. The flip-flop 41 has its Q output connected to a base of a bipolar transistor 48 to turn off in synchronism with the turning on of MOSFET 32. Included in the controller 40 is a current mirror of transistors 43 and 44 which provides a constant current set by DC supply 45 and a resistor 46 to charge a timing capacitor 47 by that current. The capacitor 47 is connected to noninverting input of a comparator 50 in parallel relation to the transistor 48. A voltage divider of resistors 36 and 37 is connected across the smoothing capacitor 34 to provide a corresponding voltage to inverting input of a differential amplifier 51 with a capacitor 53 connected between the inverting input and the output of the amplifier 51 as a feedback impedance. The differential amplifier 51 has its noninverting input connected to receive a fixed reference voltage from another DC supply 52 so as to provide a voltage which is a function of the difference between the inputs. The resulting voltage, which is substantially constant, is fed to inverting input of the comparator 50 as a threshold voltage VTH determining a timing at which MOSFET 32 is turned off. That is, the comparator 50 produces a high level output to reset input R of flip-flop 41 when capacitor 47 is charged up to the threshold voltage VTH of the comparator 50, thereby providing a low level gate signal from Q output to turn off MOSFET 32 for releasing the current from the inductor 31 to the smoothing capacitor 34 through diode 33. At the same time, flip-flop 41 provides a high level at Q output to turn on transistor 48. At this occurrence, capacitor 47 is shunted by transistor 48 now made conductive to discharge so as to be ready for being subsequently charged after the current sensor 42 provides a high level signal to set input S of flip-flop 41 in response to the current through the inductor 31 reducing to zero. In this manner, MOSFET 32 is turned on upon the current through the inductor 31 decreasing to zero and is kept turn on for a fixed time interval determined by a time constant of resistor 46 and capacitor 47.
It is noted in this connection that a response delay is inevitable in the circuit from an instant when the current through the inductor 31 reduces to zero to an instant when MOSFET 32 is actually turned on. In consideration of this delay, the operation of the prior power supply of FIG. 5 is explained in detail with reference to FIG. 6 which illustrates pulsating DC voltage from the rectifier 22, current flowing through the inductor 31, voltage developed across the capacitor 47, and a gate signal to MOSFET 32. At time t1, MOSFET 32 is turned on to begin supplying the current to the inductor 31 after the response delay from an instant when the current through inductor 31 decreases to zero, which condition is detected by current sensor 42. At the same time, flip-flop 41 responds to turn off transistor 48 to begin charging capacitor 47 by the current mirror. At time t2 where capacitor 47 is charged up to the threshold level VTH of comparator 50, the flip-flop 41 responds to the high level signal from the comparator 50 to the reset input R for turning off MOSFET 32 and at the same time turning on transistor 48, whereby allowing the inductor 31 to begin releasing the current from the inductor 31 for charging the smoothing capacitor 34, while discharging capacitor 47 through transistor 48 now made conductive. The charging of smoothing capacitor 34 continues until MOSFET 32 is again turned on with the delay from an instant when the current through inductor 31 decreases to zero. In this manner, this prior power supply can turn on MOSFET 32 dependent upon a timing when the current through the inductor 31 decreases to zero and therefore reduce the dead period in which the current through the inductor oscillates.
Nevertheless, this prior power supply still suffers from a problem in that the input current wave form is distorted or is not well conformed to the input voltage at a period Tx, as shown in FIG. 7C, which corresponds to a low level range of the pulsating DC voltage from the rectifier 22. Since the current through the inductor goes negative due to the resonant circuit inevitably formed in the chopper circuit and the response delay from the instant when the current decreases to zero and the time when MOSFET 32 is actually turned on, the inductor 31 always flows the current firstly in the negative direction. It should be noted in this connection that a gradient of the current flowing through the inductor when supplying the energy to the inductor from the rectifier is expressed by VIN/L (wherein VIN is the input voltage to the chopper CH from rectifier 22 and L is an inductance of the inductor) and the current flowing through the inductor 31 when releasing the energy from the inductor is expressed by (VOUT-VIN)/L (wherein VOUT is an output voltage of the chopper). Since VOUT is kept at substantially a constant level, it is known from the above relations that when the pulsating DC voltage from the rectifier 22 is around a high level (FIG. 7A), the supplying current increases at a great gradient and releasing current decreases at a less gradient, and that when the pulsating DC voltage is around a low level (FIG. 7B), the supplying current increases at a low gradient and the releasing current decrease at a high gradient. This means that, within the same response delay ts after the current through the inductor decreases to zero, the releasing current will flow in the negative direction to a greater extent (FIG. 7B) and increase slowly at the lower rectifier output (FIG. 7B) than at the higher rectifier output (FIG. 7A). Consequently, within the fixed ON-period T1 of MOSFET 32 the amount J1 of the current continuously flowing in the negative direction immediately after the turn on of MOSFET 32 becomes greater than the amount J2 of the current flowing in the positive direction of which duration is indicated by T2 in the figures when the rectifier output is low, as shown in FIG. 7B. In other words, the effective duration T2 in which the positive current flows through the inductor to store the energy therein reduces as the rectifier output becomes lower, thereby failing to establish the relation of J1&lt;J2. It is the duration Tx where J1.ltoreq.J2 that the input current wave is not well conformed to the input voltage to cause undesirable harmonics and results in lowered power factor. In this sense, this prior power supply is also found not to be successful in eliminating undesired harmonics particularly around the low level output of the rectifier to the chopper and therefore in improving the power factor.